(anatomy) a natural hollow or sinus within the body

membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo

membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional

thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)

the outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals)

the saclike body cavity of a coelenterate